An organic electroluminescent (OEL) device, alternately known as organic light emitting diode (OLED), is useful in flat-panel display applications. This light-emissive device is attractive because it can be designed to produce red, green, and blue colors with high luminance efficiency; it is operable with a low driving voltage on the order of a few volts, and clearly viewable from oblique angles. These unique attributes are derived from a basic OLED structure comprising of a multilayer stack of thin films of small-molecule organic materials sandwiched between an anode and a cathode. Tang et al in commonly-assigned U.S. Pat. Nos. 4,769,292 and 4,885,211 have disclosed such a structure. The common electroluminescent (EL) medium is comprised of a bilayer structure of a hole-transport (HTL) layer and an electron-transport layer (ETL), typically on the order of a few tens of nanometer (nm) thick for each layer. When an electrical potential difference is applied at the electrodes, the injected carriers-hole at the anode and electron at the cathode-migrate towards each other through the EL medium and a fraction of them recombines in the emitting layer (EML) a region close to the HTL/ETL interface, to emit light. The intensity of electroluminescence is dependent on the EL medium, drive voltage, and charge injecting nature of the electrodes. The light viewable outside of the device is further dependent on the design of the organic stack and optical properties of the substrate, anode and the cathode.
Conventional OLEDs are bottom emitting (BE), meaning that the display is viewed through the substrate that supports the OLED structure. The devices normally employ transparent glass substrates having a layer of highly transparent indium-tin-oxide (ITO) layer that also serves as the anode. The cathode is typically a reflective thin film of MgAg, although lithium-containing alloys are also used as an efficient electron-injecting electrode. The light generated within the device is emitted in all directions. However, only a small fraction of generated light is available for viewing, and about 80% of generated light is trapped within the device in waveguiding modes in glass, ITO and organic layers. The light emitted toward the anode at less than the critical angle passes through the anode and through the substrate to the viewer, and the light emitted in the opposite direction is reflected at the cathode and passes through the substrate, enhancing the viewing intensity. A transparent substrate, a high-transparency anode and a high-reflectivity cathode are required to yield high luminance efficiency devices.
The OLED display is typically coupled with active matrix (AM) circuitry in order to produce high performance displays. For the AM bottom emitting display, which uses switching elements of thin film transistors, the transistors are fabricated on glass substrates. Consequently the open area available for the light to emerge is reduced. With the application of multi-transistor and complex circuitry in the backplane the open area through which the light emerges is reduced. The ratio of the open area to that of the entire display area is called the aperture ratio. Due to the reduction of the aperture ratio the display will run dim. To compensate for the reduced average brightness level the drive current has to be increased subjecting the display to increased risk of operational degradation. It follows that more complex pixel drive circuitry cannot be readily implemented without further compromising the aperture ratio and the operational stability.
To alleviate this problem the emitted light can be made to emerge through the top surface. In the top-emitting design the drive circuitry is fabricated on substrate and the light emerges through the opposite surface. This design permits the use of complex circuitry occupying whatever substrate space is needed and the light-emitting area of the cathode and hence the aperture ratio is not affected. The high aperture ratio makes the display viewable consuming less power. The devices have the prospect of running at low drive current while maintaining readability and extending their operational life.
Devices employing opaque backplanes such as silicon the OLED must be of the top-emitting type. The top surface, usually the cathode, needs to be at least semitransparent in order to allow the light to exit through the top. The device should preferably include a reflector or a reflecting anode opposite to the cathode side to redirect the light that strikes the anode to the cathode side.
Any device design should be aimed at achieving highest possible efficiency. However, realizing high efficiency by reclaiming light lost to waveguiding modes can be very difficult. To recover even a fraction of light lost to the waveguiding modes the device architecture can be very complex.
An approach to enhance the efficiency without introducing such complexity is to implement a microcavity design for the device, which includes reflecting electrodes. By employing highly reflective electrodes it is possible to remarkably increase the out-coupling of generated light. Sony Corporation (EP 1 154 676 A1) has disclosed an anode made of light-reflecting materials such as Pt, Au, Cr, W, or presumably other high-work function materials in conjunction with an optional buffer/hole-injecting layer (HIL). Sony also has disclosed (EP 1 102 317 A2) that an anode composed of a transparent conducting film such as ITO formed on the reflecting layer. The top electrode was a semitransparent reflecting layer of MgAg or Al:Li alloy serving as the cathode through which the light emerges. Lu et al. (“High-efficiency top-emitting organic light-emitting devices”, M.-H. Lu, M. S. Weaver, T. X. Zhou, M. Rothman, R. C. Kwong, M. Hack, and J. J. Brown, Appl. Phys. Lett. 81, 3921 (2002)) disclosed a top-emitting, highly efficient OLEDs that used reflective metals in the anode structure, a phosphorescent emissive layer, Ir(ppy)3, and a semitransparent compound cathode. Riel et al. (“Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling”, H. Riel, S. Karg, T. Beierlein, B. Rushtaller, and W. Rieb, Appl. Phys. Lett. 82, 466 (2003) demonstrated a high-efficiency top emitter, also using the Ir(ppy)3 emissive layer, high work-function metal anodes, and semitransparent metal cathodes and further employing a ZnSe capping layer over the semitransparent compound cathode for improved light outcoupling. These top-emitters demonstrated efficiencies that are higher than the equivalent bottom-emitting non-microcavity devices.
Although microcavity devices can be highly efficient, the emission from microcavity devices is characteristically directional. The microcavity device can cause color distortion when viewed at oblique angles. The emission shifts to shorter wavelength and the intensity falls rapidly with viewing angle (“Control of emission characteristics in organic thin film electroluminescent diodes using an optical microcavity structure” N. Takada, T. Tsutsui, and S. Saito Appl. Phys. Lett. 63 (15) 2032 (1993)). It is shown that for a device with a reflective MgAg electrode and a semitransparent Ag electrode the peak wavelength shifted progressively toward the shorter wavelength side with increasing detection angle. The shift in peak wavelength at 45 degree was reported to be about 50 nm at a detection angle of 45 degree whereas a comparable non microcavity device exhibited negligible wave length shift. The microcavity devices thus are expected to exhibit significant color change with viewing angle. Raychaudhuri et. al. (“Performance enhancement of bottom-and top-emitting organic light-emitting devices using microcavity structures”, P. K. Raychaudhuri*, J. K. Madathil, Joel D. Shore and Steven A. Van Slyke, Proceedings of the 23 rd International Display Research Conference, Phoenix, Ariz., Sep. 16 to 18, 2003, p 10) discussed monochrome microcavity devices that are twice as efficient as optimized bottom-emitting non-microcavity devices and exhibiting negligible wavelength shift with viewing angle. The thickness of the hole-transport layer, and hence the cavity length was very precisely adjusted to minimize the angular dependence. Such tight thickness control in manufacturing is difficult and may be practically unattainable.
Tam et. al (“Active Textured Metallic microcavity” H. L. Tam, R. Huber, K. F. Li, W. H. Wong, Y. B. Pun, S. K. So and K. W. Cheah, Proceedings of the 7th Asian Symposium on Information Display (ASID 02) Singapore 2002, p 453) discussed a textured metallic microcavity having a two-dimensional wavelength scale periodic structure with nanometer precision. On a plane semitransparent Ag electrode a periodic pattern was created by electron-beam lithography upon which an emitter and the semitransparent Ag electrode was deposited in sequence resulting in a microcavity structure with a textured mirror. Their results from angle resolved transmission experiments show that the photo luminescence (PL) peak shifts with the viewing angle from the textured microcavity is less when compared with that from a planar microcavity sample. This type of device structure in manufacturing environment is most likely undesirable.
A top-emitting planar microcavity OLED is expected to exhibit color distortion with viewing angle. A semitransparent metallic cathode can have sufficient residual reflection to cause microcavity effect that distorts the emission color. (“Transparent stacked organic light emitting devices. I. Design principles and transparent compound electrodes”, G. Gu, G. Parthasarathy, P. E. Burrows, P. Tian, G. Hill, A. Kahn, and S. R. Forrest, Appl. Phys. Lett. 86 (8) 4067 (1999)). However when a highly transparent metal free cathode of indium-tin-oxide (ITO) was substituted for the semitransparent cathode the angular dependence of emission colors due to microcavity effect became weak. (“A metal-free full-color stacked organic light-emitting device”, G. Gu, G. Parthasarathy, and S. R. Forrest, Appl. Phys. Lett. 74 (2) 305 (1999)).
In top-emitting OLED the top electrode, usually a cathode, includes a low work function metal or a metal on an electron-injecting surface. To achieve high efficiency the transparency of the cathode needs to be high requiring the use of thinnest possible layer. But such thin films are not sufficiently electrically conductive making the implementation in large display difficult because of the greater distance that the current must travel. A capping layer of conductive and highly transparent material is needed to increase lateral electrical conductivity without significantly decreasing the light output. Additionally, the capping layer helps to preserve the integrity of the cathode. Indium tin oxide (ITO) is the most commonly used transparent conducting oxide (TCO). However, deposition methods for this material are not compatible with that of the organic layers of OLEDs. Deposition methods for TCOs generally involve sputter deposition. Sputtering is a preferred method as it permits optimization of the film composition during film deposition for maximization of transparency and conductivity. However, sputtering deposition directly on the electron transport layer (ETL) can result in degraded device performance. An Ar plasma commonly employed in sputtering is known to cause severe degradation of Alq, a widely used electron transport material. (“Ion-beam induced surface damages on tris-(8-hydroxyquinoline) aluminum”, L. S. Liao, L. S. Hung, W. C. Chan, X. M. Ding, T. K. Sham, I. Bello, C. S. Lee, and S. T. Lee, Appl. Phys. Lett. 75, 1619 (1999)). This damage reduces the intensity of the emission and may additionally permanently damage the pixels. Thus, it is necessary to protect the ETL during sputtering deposition of TCOs, and thin cathode layer, typically of the order of 10-nm may not be adequately effective.
In U.S. Pat. No. 6,420,031, assigned to The Trustees of Princeton University, a class of low reflectivity, high transparency, non-metallic cathodes useful for a wide range of electrically active, transparent organic devices is disclosed. The representative embodiment of this invention employs ITO as the electrically conductive non-metallic layer and a phthalocyanine compound such as ZnPc or CuPc as the electron-injecting interface layer. The low-resistance electrical contact is formed when the ITO is deposited onto the organic layer. The CuPc layer functions as: 1) a protection layer, preventing damage to the underlying organic layers during the ITO sputtering process; and 2) an electron-injecting region, functioning in combination with the ITO layer to deliver electrons to the adjacent electron transporting layer. This solution for delivering a highly transparent cathode for use in an OLED is insufficient as the buffer materials may be unsuitable for full color devices.